In a process referred to as quorum sensing, bacteria communicate using chemical signal molecules called autoinducers. By monitoring increases and decreases in autoinducer concentration, quorum-sensing bacteria track changes in cell-population density and synchronously switch into and out of group behaviors. Quorum sensing allows bacteria to collectively carry out tasks that would be unsuccessful if carried out by an individual bacterium acting alone.
Both Gram-positive and Gram-negative infectious bacteria, which include human, animal, plant, and marine pathogens, use quorum sensing strategies to control virulence. Typically, bacterial infections are treated with bactericidal or bacteriostatic molecules that impede four major processes: DNA replication, transcription, translation or tetrahydrofolic acid synthesis. Existing methods for treating bacterial infection unfortunately exacerbate the growing antibiotic resistance problem because they inherently select for growth of bacteria that in turn can resist the drug. What is needed are new treatments that avoid selecting for drug resistant bacteria.
Quorum sensing also controls biofilm formation. Biofilms are communities of bacterial cells adhered to surfaces and are highly problematic, for example in industrial processes (e.g., clogging of cooling towers in manufacturing plants) and in hospital or other clinical settings (e.g., catheter and implant infections). Initial studies with Staphylococcus aureus and Staphylococcus epidermidis indicated that manipulation of a form of quorum sensing that is peptide-mediated would not have successful results. Most notably, disruption of the peptide quorum-sensing circuit in S. epidermidis by deleting necessary quorum sensing genes led unexpectedly to increased biofilm formation on implanted medical devices. Therefore what is needed are new treatments for bacterial infection that can more subtly manipulate bacterial behaviors that promote health problems.
The bacterium Pseudomonas aeruginosa is the major pathogen associated with cystic fibrosis lung infection, keratitis eye infection, and third-degree burn-associated skin infections. P. aeruginosa has a complex signaling pathway that governs quorum sensing and virulence (FIG. 1B). The signaling pathway includes LasI, a synthase enzyme that makes the native acyl-homoserine lactone (AHL) signal, 3OC12-HSL. The native signal is detected by the transcriptional regulator LasR, forming a LasR:3OC12-HSL complex. The LasR:3OC12-HSL complex affects gene transcription, turning on virulence factors, the Rhl system and additional quorum-sensing circuits.
Another synthase, RhlI, produces another AHL (C4-HSL), which is detected by the transcriptional regulator RhlR. The RhlR:C4-HSL complex also regulates virulence genes and other components of the signaling pathway. Virulence production is impacted by multiple other factors, including the transcription factor QscR and the PQS system that produces and detects quinolone signals.
This tandem regulatory arrangement allows LasI/R to control the first wave of quorum-sensing-controlled gene expression and RhlI/R to control the second. Because LasR activates expression of rhlR, deletion of lasR reduces expression of both LasR- and RhlR-regulated target genes.
Additionally one key factor in pathogenicity of a bacterial infection is the production of virulence factor produced at high cell density, such as pyocyanin. This small molecule is redox active and is important for maintaining the redox balance in P. aeruginosa, particularly under low oxygen or anaerobic conditions. RhlR is a key transcriptional regulator controlling the up-regulation of the pyocyanin biosynthetic pathway, which in turn is induced by the LasR:3OC12-HSL complex (FIG. 1B). Thus, new compounds and methods of treating bacterial infection and/or contamination are needed.